1. Field of the Invention
The present invention relates to a memory controller, a memory system, a recording and reproducing method for the memory system, and a recording apparatus, more particularly, to a memory controller configured to control a memory system that records a data sequence in a memory unit including memory cells, each of which assumes a record state according to data and reproduces the recorded data sequence; a memory system; a recording and reproducing method for the memory system; and a recording apparatus.
2. Description of the Related Art
Memory systems, particularly memory systems configured to record data in an NAND-type flash memory, a non-volatile recording medium, have been developed. The memory systems have been widely used as memory devices of host devices in a personal computer, a mobile phone, a digital camera and the like, or build-in memory systems in the computer systems. Among others, recently, the memory systems having the NAND-type flash memory have been particularly widely used.
In a record operation, the NAND-type flash memory injects a predetermined amount of charge into a charge accumulation layer of a memory cell in a semiconductor memory unit through an insulating film according to data to be recorded. In a reproducing operation, the NAND-type flash memory measures a difference between threshold voltages of transistors according to the amounts of charge in the memory cells and reads out the recorded data. Here, the memory cells assume a plurality of record states with different threshold voltages. Corresponding data are determined for respective record states. In two-value memory cells, for example, a memory cell in a low threshold voltage state with no charge accumulated is determined as “1” data, while a memory cell in a high threshold voltage state with a charge accumulated is determined as “0” data.
The memory cell with no charge accumulated and the memory cell with a charge accumulated, in other words, the memory cells with different threshold voltages differ in their physical stable states. For the known memory systems, however, which type of data should correspond to one of the respective record states of the memory cell has not been considered.
Japanese Patent Application Laid-Open Publication No. 2001-93288 discloses a multivalued flash memory device capable of recording 2-bit data in one memory cell by using record states with four different threshold voltages. Japanese Patent Application Laid-Open Publication No. 2001-93288 also discloses that the multivalued flash memory device requires a longer time in accumulating a charge in a memory cell to a high threshold voltage state than in accumulating to a low threshold voltage state.